1. Field of the Invention
This invention relates generally to the field of cathode ray tubes, and, in particular, to detecting breakdown events in cathode ray tubes having a focus mask color-selection structure.
2. Description of Prior Art
In a video display apparatus, such as a television receiver or a computer or video display terminal, a cathode ray tube (CRT) includes an evacuated envelope usually made of high-strength glass. The envelope includes a generally flat or slightly curved faceplate together with a funnel-shaped bell and extending neck. The interior side of the faceplate supports a phosphor screen.
In a color CRT, a plurality of electron guns are used together with a phosphor screen which supports plural areas of phosphors having differing color light-emitting characteristics. When an electron beam strikes the phosphor screen, visible light is emitted therefrom. A color-selection structure is interposed between the electron guns and the phosphor screen to cause each of the electron guns to strike only an associated type of colored light-emitting phosphor.
One such structure is a shadow mask. A shadow mask is a thin sheet of steel having a plurality of apertures through which electrons must pass on their way to striking the phosphor screen. The shadow mask functions as a filter in that only those electrons incident to the shadow mask at the appropriate angle will pass through its apertures and strike the phosphor screen at the appropriate location.
One disadvantage of the shadow mask is that it is only approximately 20% transmissive, meaning simply that only approximately 20% of the electrons emitted by the CRT's electron guns eventually pass through the shadow mask's apertures and strike the phosphor screen. The remaining electrons will be absorbed by the shadow mask, where their energy will be dissipated as heat. A shadow mask has a maximum theoretical transmissivity of approximately 33% and a typical transmissivity that is equal to approximately 18%.
Several techniques are known to the art for increasing the transmissivity of a color-selection structure while still ensuring that the electrons passing through the color-selection structure excite only an associated type of colored light-emitting phosphor on the screen. One such technique employs a two-layer focus-mask color-selection structure to define a quadrupole electrostatic lens in each of the apertures of the color-selection structure. Each quadrupole lens focuses the electron beamlets passing through that quadrupole lens in one transverse direction and defocuses them in the orthogonal transverse direction on the target according to the relative magnitudes and polarities of the electrostatic fields comprising the quadrupole lens. The use of a focus mask structure permits an electron transmissivity in excess of approximately 60%, and focus mask structures have a maximum theoretical transmissivity that approaches unity.
Focus mask CRTs have been successfully constructed, but a critical operational defect has been noted. Specifically, experimental focus mask CRTs have been found to develop anomalies, which are best described as discoloration bands, that extend horizontally across the screen. These discoloration bands can occur quite frequently and can significantly impair the utility of CRTs employing focus-mask type color selection means.